Increasing atmospheric carbon dioxide (CO2) is raising seawater CO2 concentrations and thereby acidifying ocean water. But a second environmental problem, eutrophication, is also causing large CO2 inputs into coastal waters. This occurs because anthropogenic inputs of nutrients have fueled massive algal blooms, which deplete bottom waters of oxygen (O2) and release CO2 when the organic matter from these blooms is respired by bacteria. Based on a biogeochemical model, these CO2 inputs are predicted to decrease current pH values by 0.25 to 1.1 units, effects that increased with decreasing temperature and salinity. Our model predictions agreed well with pH data from hypoxic zones in the northern Gulf of Mexico and Baltic Sea, two eutrophic coastal systems with large temperature and salinity differences. The modeled and measured decreases in pH are well within the range shown to adversely impact marine fauna. Model calculations show that the acidification from respiratory CO2 inputs interacts in a complex fashion with that from increasing atmospheric CO2, and that these pH effects can be more than additive in seawater at intermediate to higher temperatures. These interactions have important biological implications in a future world with increasing atmospheric CO2, increasing anthropogenic inputs of nutrients, and rising temperatures from CO2-linked global warming.

Sunda W. G. & Cai W.-J., in press. Eutrophication induced CO2-acidification of subsurface coastal waters: interactive effects of temperature, salinity, and atmospheric PCO2. Environmental Science & Technology. doi: 10.1021/es300626f. Article (subscription required).